When Jones retired in 1988, Tanner won the Democratic nomination for the seat and handily defeated Republican nominee Ed Bryant, who went on to represent the neighboring 7th District from 1995 to 2003. Tanner was reelected in 1990 with no major-party opposition, a feat he repeated in 1992. In 1998, he was completely unopposed. He handily defeated Republican opponents in 1994 (the only time besides his initial election that he faced a serious or well-funded challenger), 1996, 2000, 2002, 2004 and 2006. In 2004 and 2008, he faced James L. Hart, an avowed eugenicist. On both occasions, Hart was disavowed by the state Republican Party.

Tanner announced on December 1, 2009 that he would not seek re-election in 2010 and would retire after 11 terms in Congress.[2]

Tanner is strongly in favor of balancing the budget and paying down the national debt. He has been a strong opponent of the fiscal policies of President George W. Bush, voting against many of the tax cuts passed during his terms; yet, he was one of 43 Democrats to vote to repeal the estate tax in 2006. Tanner was one of the few Democrats in the House to vote in favor of CAFTA and has long distanced himself from the majority of his party on issues such as bankruptcy law and lawsuit reform. He voted for the Federal Marriage Amendment, the ban on "partial-birth" abortions, limiting death penalty appeals, and has voted against most gun control measures. On other issues he is more liberal: he often votes with his party on separation of church and states issues, and has consistently voted against the Flag Desecration Amendment. Tanner voted with the majority of his party to expand stem cell research and against renewing the controversial portions of the Patriot Act. He also supports affirmative action and public education. Tanner was firmly opposed to Bush's attempt to reform Social Security.

Tanner received much of his knowledge of politics as a youth from his father E.B. "Buzz" Tanner who was successful in the financial and insurance business. He became a member of a highly political and influential law firm out of law school in his home town community and soon ran for office as a state legislator in the House of Representatives. He was also a relative of then House Speaker Ned McWherter, who later was elected governor twice and was an extremely powerful political force in the state, having friends on both sides of the political aisle.

In 2004, Congressman Tanner made a brief but unintended cameo appearance alongside Michael Moore in Fahrenheit 9/11, where Moore was trying to get Congressmen to have their children enlist in the Military to go to Iraq.

He recently drafted a bill that would require special bipartisan commissions rather than state legislatures to redraw congressional districts when necessary due to U.S. Census count changes. It is generally believed that this bill is a response to Republican-inspired mid-decade redistricting in Texas and recent similar efforts in Colorado and Georgia.

After both his district and state chose the former first lady,[3] Congressman Tanner endorsed Senator Hillary Clinton's presidential campaign in April 2008.[4]

He currently works for Nashville law firm Miller & Martin and the Prime Policy Group, a lobbying firm.[5] He became chairman of the International Conservation Caucus Foundation, a non-profit group that advocates U.S. leadership in conservation programs globally, in fall of 2011.